Within the United States, digital radio broadcasts protocols are based upon the HD (High Definition) Radio standard developed by iBiquity Digital Corporation. For current HD Radio broadcasts, digital content is broadcast in upper and lower sidebands related to a center frequency for a primary analog AM or FM broadcast channel. These upper and lower sidebands include ten (10) frequency partitions which include digital channels that are modulated using OFDM (orthogonal frequency division multiplexing) and are further shaped using a pulse shaping function prior to transmission.
In other parts of the world including countries within Europe, one or more DAB (Digital Audio Broadcasting) standards are used for broadcasting of digital audio content. The DAB standards also use OFDM modulation, and DAB broadcast channels include digital data within a large number of OFDM subcarriers that are modulated using DQSPK (differential quadrature phase shift keying) modulation. For example, Transmission Mode I (TM-I) for the DAB standard includes 1536 OFDM subcarriers each having a bandwidth of 1 kHz (kilo-hertz) within an overall OFDM channel having a bandwidth of 1.537 MHz (mega-Hertz). Each OFDM symbol for the TM-I DAB transmission has an overall symbol time of 1.246 milliseconds (ms) including a guard interval of 0.246 ms within which a cyclic prefix is transmitted and including a data region of 1 ms within which useful data is transmitted.
FIG. 1A (Prior Art) provides an embodiment 100 for a DAB channel 102 centered on a channel center frequency (fcH) 104. The DAB channel 102 includes 1536 OFDM subcarriers that are each modulated to include a portion of the broadcast data. For one embodiment as indicated above, each of these subcarriers can be 1 kHz wide and the overall bandwidth of the OFDM channel can be 1.537 MHz.
FIG. 1B (Prior Art) is an embodiment 150 for an OFDM symbol 152 associated with the DAB channel 102 in FIG. 1A (Prior Art). The OFDM symbol 152 has a guard interval 154 including a cyclic prefix and a region 156 including useful data for the OFDM symbol 152. The useful data region 156 includes M samples, and the cyclic prefix within the guard interval 154 includes a copy of the last L samples of the useful data region 156. As such, the guard interval 154 can be considered to include samples M−L to M−1, and the useful data region 156 can be considered to include samples 0 to M−1. The symbol time 158 for the OFDM symbol 152 represents the duration in time for the transmission of the OFDM symbol.
For DAB broadcast channels, a transmission frame is commonly used that includes one (1) null symbol having a symbol time of 1.297 ms followed by seventy-six (76) OFDM symbols each having a symbol time of 1.246 ms. As such, this example DAB transmission frame has an overall duration of 96 ms. Further, for this example DAB transmission frame, no power is transmitted during the null symbol, and the first OFDM symbol after null symbol is configured as a TFPR (Time Frequency Phase Reference) symbol. This TFPR symbol can be, for example, a pre-determined fixed pattern that is known by the receiver and used by the DAB receiver for synchronization.
Prior DAB receivers are typically configured to detect the null symbol within the DAB transmission frame and then to synchronize to the DAB transmission by correlating the received TFPR symbol with the ideal, expected TFPR symbol. However, this prior detection/synchronization technique can be relatively slow and thereby lead to a degraded and/or unsatisfactory user experience.